There have been a number of attempts to regulate metals emissions. Under Title III of the Clean Air Act Amendments of 1990, the Maximum Achievable Control Technology (MACT) standards were established. The standards identify eleven metals in the list of hazardous air pollutants (Federal Register, volume 64, No. 189, page 52845 (Sep. 30, 1999). Six of these metals are sorted into volatility groups: high volatile (mercury), semi-volatile (lead, cadmium) and low volatile (arsenic, beryllium and chromium). The remaining five metals are controlled as “particulate matter”. MACT standards have been developed both for existing and new point sources. A point source is any discernible, confined and discrete conveyance from which pollutants are or may be discharged. In principle, MACT provides a technology-driven set of federal regulations. In practice, many industries have been unable to meet the proposed limits by using currently available pollution control technologies.
Mercury from anthropogenic sources presents a difficult environmental problem. In comparison to other toxic metals, mercury has a very low vaporization temperature. Mercury and mercury compounds are highly toxic, and organic forms such as methyl mercury can be bio-accumulated. Exposure pathways include inhalation and transport to surface waters. Mercury poisoning can result in both acute and chronic effects. Most commonly, chronic exposure to mercury vapor affects the central nervous system and the brain, resulting in neurological damage.
Mercury speciation is an important factor in appropriate removal strategies. Mercury can exist in the elemental form (Hg0) and in ionic forms (Hg+2, Hg2+2). Speciation is highly dependent upon the chemical environment. Mercury speciation in the atmosphere is typically quite different than speciation from anthropogenic sources. In flue gas, mercury primarily exists in the ionic form, mainly as HgCl2 and also as HgO. On average, the speciation of mercury in flue gases is 79% ionic, with the balance elemental mercury. Understanding and controlling the chemistry is critical to proper pollution control. For example, electrostatic precipitators can be very effective in removing ionic mercury, but inefficient in removing elemental mercury.
The flue gas composition can strongly influence the success of a mercury capture technology. Typical flue gas components can include O2, HCl, Cl2, SO2, NO2, N2O, NO, NH3 and H2S. The presence and concentrations of these species can greatly enhance or complicate the mercury capture process. For example, HCl can result in the formation of HgCl2 that can be scrubbed or absorbed. Alternatively, other species can act as reducing agents, with the undesirable outcome of converting ionic or oxidized mercury to elemental mercury.
There are a number of mercury removal technologies available. Such technologies include adsorption, amalgamation, wet scrubbing and electrostatic precipitation. However, the Environmental Protection Agency has concluded that no single technology has proven efficient for mercury capture (EPA-452/R-97-003).
Carbon filters function via adsorption and are used to remove organics, heavy metals and acid gases. Oxidized mercury is captured by activated carbon while elemental mercury is not. Activated carbon injection into hot flue gas has also been explored. This is limited to low temperature operations for effective removal (<90-120° C.; 200-250° F.). Packed beds of sulfur, iodine or chlorine impregnated carbon have also been utilized.
Numerous other sorbents have also been tested. These are discussed in detail by Granite et al. (2000). A method for in situ generation of sorbents has also been disclosed in U.S. Pat. No. 5,888,926. In general, mercury capture by sorbents is mass transfer limited. This is due to the very low mercury concentration in high volumes of flue gas. This results in competition with other species for active sites on the sorbent. Uncertainties also exist regarding the sufficiency of residence time in various applications. Spent carbon must also be disposed of, has a finite adsorption capacity, and can potentially lead to bed fires resulting from hot spot formation. Disposal options include combustion, landfilling, or treatment as a hazardous waste.
Wet scrubbing is used as a gas treatment scheme to remove acid gases, metals, particulate matter, dioxins and furans. However, this is a very limited method for mercury capture due to the near insolubility of mercury and mercury oxide.
Selenium filters have been tested on flue gas streams with low Hg concentrations. However, filter lifetime is limited, the selenium filter is not regenerable, and the HgSe formed must be landfilled. Other regenerable noble metal sorbents, such as gold monoliths have been developed (see U.S. Pat. No. 5,409,522).
A method for removing elemental mercury from a gas stream by an oxidation reaction to form a water-soluble mercury compound has been disclosed in U.S. Pat. No. 5,900,042. Aqueous iodine, bromine, chlorine and chloric acid are described for reaction with mercury to form soluble halogenated mercury compounds. The '042 patent discloses the injection of a reactive solution into the gas duct using a nozzle or an atomizer to generate a mist. Alternatively, the gas stream may be contacted with the reactants in a liquid scrubber. Reported test results varying bubble size indicated that gas phase reactions are particularly important for complex flue gas mixtures. The test results also indicate that the reactions may be kinetically or mass transfer limited, as mercury removal is less than optimal.
The chemistry of metal perchlorates has been reviewed by Gowda et al. (1984) and Pascal and Favier (1998). Gowda et al. state that a “considerable” number of mercury perchlorate complexes are known, including a number of complexes containing organic molecules. Pascal and Favier discuss synthetic methods, including the use of HClO4 and Cl2O6 as starting materials. Fourati et al. (1987) used chlorine trioxide (Cl2O6) to synthesize a highly ionic compound HgCl(ClO4). Other compounds identified and synthesized include a mercury (I) perchlorate, Hg2(ClO4)2, a mercury (II) perchlorate, Hg(ClO4)2, a mercury (II) oxide perchlorate, Hg2(ClO4)2.2HgO (Nikitina and Rosolovskii, 1986) and hydrated mercury perchlorates.
Oxygen-chlorine reactions are particularly important in atmospheric chemistry. The experimental and theoretical literature in this area is substantial and useful in predicting and understanding the pertinent gas and liquid phase chemistry. The atmospheric chemistry of mercury and reactions with HOCl/OCl− have been detailed (Lin and Pehkonen, 1998, 1999). Ab initio methods have been utilized to investigate the gas phase properties of potentially important reactants such as O+OClO (Colussi et al. 1992) and HClO3 (Francisco and Sander, 1996). The equilibrium structures HOClO3 and HO4Cl have also been examined (Francisco, 1995). This work determined that in addition to perchloric acid (HOClO3), a linear chain HOOOOCl isomer (bonded dimer of HO2 and ClO2) is a stable structure.
Non-thermal atmospheric pressure plasma systems have been demonstrated for emissions reductions by a number of different researchers. Such work has primarily targeted NOx control (Penetrante et al. 1999). These plasmas produce highly reactive ions and metastable species to achieve chemical and thermal conversions, with gas temperatures on the order of 100° C. Similar devices have also been demonstrated for destruction of low concentrations of volatile organic compounds such as dichloromethane, methyl chloride, carbon tetrachloride, trichloroethane, trichloroethene and chlorobenzene (Fitzsimmons et al. 2000). A limited amount of research has also discussed volatile metals capture. Non-thermal plasma-based devices for mercury removal have exclusively utilized an oxygen based, barrier discharge type plasma. The chemistry employed in these devices is the reaction of activated oxygen with elemental mercury to form mercury (II) oxide. This oxide particle is captured downstream using conventional means.
U.S. Pat. No. 6,117,403 discloses an atmospheric pressure corona discharge oxygen device that can be used for mercury removal. Testing on flue gases with this system is further described in McLarnon et al. (2000). The device includes initial particulate removal in a dry electrostatic precipitator, conversion of elemental mercury to HgO via an oxygen plasma and subsequent collection on a wet electrostatic precipitator. The reported mercury removal efficiencies range from 68-82%. The upstream filtering device is primarily used to prevent electrode fouling. However, this precludes the participation of the initial particulates in a downstream agglomeration and growth scheme.
U.S. Pat. No. 5,785,932 reports a process employing the combination of a perforated corona discharge plate and catalyst. In the absence of the corona discharge, the catalyst functions to adsorb elemental mercury from the gas stream. When the corona discharge device is energized, the molecules desorb and are oxidized. These mercuric oxide particles are captured using conventional particulate control technologies. The reactor relies upon the development and highly efficient function of a catalyst containing vanadium and titanium for the adsorption of elemental mercury. Details of the catalyst adsorption characteristics including mercury competition with other off-gas components are not provided.
The presence of larger particulates may prove beneficial to promote particle clustering or agglomeration. For example, such primary large particles may serve as growth sites for mercury compounds. A similar concept has been reported after plasma treatment of simulated engine exhaust gases (Hoard et al. 2000).
A number of atmospheric pressure plasma devices are disclosed. U.S. Pat. No. 5,414,324 describes the design of a One Atmosphere Uniform Glow Discharge Plasma device. U.S. Pat. No. 5,961,772 discloses the design for a non-thermal atmospheric pressure plasma jet. This device reports the use of activated species generated using plasma gas mixtures of CF4/O2/He, O2/He and O2/H2O/He. U.S. Pat. No. 5,977,715 describes a handheld glow discharge atmospheric pressure plasma device producing plasma comprised of mixtures of Ar, He and O2. U.S. Pat. No. 6,030,506 describes the generation of non-thermal plasma species introduced into a fluid medium by high-speed injection. The activated species described include monatomic nitrogen and oxygen, OH., H2O., SH., CH3., and other hydrocarbon species.
It must be noted that the prior art referred to hereinabove has been collected and examined only in light of the present invention as a guide. It is not to be inferred that such diverse art would otherwise have been assembled absent the motivation provided by the present invention, nor that the cited prior art when considered in combination suggests the present invention absent the teachings herein.
It would be desirable to produce a plasma based trace metal removal apparatus and method for the capture and removal from gas streams of mercury and other volatile and semi-volatile metals and trace species.